1. Field of the Invention
The present invention relates to a surge protection device and more particularly to a surge protection device providing surge protection for back-end circuit and itself.
2. Description of the Related Art
To protect power supply equipment and a load receiving power from the power supply equipment against the damage caused by lightning, a surge absorber is traditionally connected to a power line loop. The surge absorber is mainly built with a metal oxide varistor (MOV) and serves to absorb enormous energy generated upon discharge of lightning. As specified in UL1449 safety standards, a surge protection device should be tested with an abnormal voltage in a range of 240 VAC, 0.2 A˜1000 A. Under the condition of the abnormal voltage, the MOV is short-circuited due to thermal activation and the huge current passing through the MOV causes the MOV to generate high heat. There is a likelihood that the MOV may explode or burn to put surrounding equipment and humans in jeopardy. Another potential risk is that when failing to withstand high voltage, such as 220 volts, the back-end circuit may also explode and burn to endanger equipment and humans nearby.
Therefore, the MOV itself needs protection too. The protection means for a conventional MOV is to incorporate a thermal fuse with the MOV. With reference to FIG. 7, given a power line loop with a live line and a neutral line as an example, a thermal fuse 81 is connected in series with the live line of the power line loop, and an MOV 82 is connected between the live line and the neutral line and is connected to a back end of the thermal fuse 81. The MOV 82 generates excessive heat when getting short-circuited and the excessive heat further melts the thermal fuse to disconnect with the power line loop. However, as the MOV 82 is suddenly activated and heat generated by the MOV 82 is unable to be promptly dissipated due to thermal resistance, the MOV 82 has usually exploded and caught fire during the course of the thermal fuse 81, and the back-end circuit often encounters damage as well. As a consequence, the surge protection circuit having the MOV 82 and the thermal fuse 81 fails to be operational again after the activation of the MOV 82.
It is also known that surge protection devices can be improved by having a switching element connected to the power line loop and controlled by a voltage detection circuit. When the voltage detection circuit detects an abnormal voltage or a surge, a switch originally in a closed state in the power line loop is switched to an open state to disconnect with the power line loop, thereby avoiding lightning surge or abnormal voltage to enter the back-end circuit of the surge protection device. The disadvantage of such technique resides in that switch bounce may occur at an operating point for protection which will jeopardize the back-end circuit.
In view of the switch bounce issue at the operating point for protection, a feasible countermeasure is to add an over-voltage protection circuit with hysteresis and set up a protection voltage and a recovery voltage. When an abnormal voltage exceeds the protection voltage, the power line loop is disconnected. After the voltage drops to the recovery voltage, the power line loop is connected again. Such approach can indeed effectively protect against the condition of long-time abnormal voltage with little fluctuation. However, to tackle the issue that the voltage fluctuates excessively or transient surge occurs from time to time, such approach has its limitations.